Real time seismic telemetry systems require a plurality of data acquisition units (DAU) to be disposed along a seismic line for sensing seismic waves in the earth's crust. Normally, these seismic waves are artificially induced to create a wavefront that can be measured as it passes proximate the DAU's. The data that is collected is normally in analog form which is obtained from a transducer such as a seismometer. Each of the DAU's is oriented such that they take data at a plurality of points disposed remote from a central shock creating device. By examining this data, scientists can utilize analytical models of the structure of the substrata in the locale of the transducers.
In operation, the seismic telemetry system is controlled such that the DAU's are normally in a stand-by mode. When a test is to be performed, commands are transmitted from a central unit to each of the DAU's and they are "powered up". They then sample the data and transmit it along a data path to the central data receiver for processing thereof. Most telemetry systems of this type utilize high frequency radio transmission techniques for transmission through the environment. Therefore, each of the DAU's has an associated path loss between its location and the central receiving unit.
To accommodate for the varying distances between each of the DAU's and the central receiving unit, the central receiving unit is provided with a wide dynamic range to account for the various power levels that are received. The DAU disposed nearestmost the central receiver has the least transmission path loss and the DAU disposed farthest most from the central receiving unit has the largest path loss. Therefore, if the power transmitted from each of the DAU's is identical, the received power from the nearestmost DAU will be higher than the received power from the farthestmost DAU by the difference in path loss between the two. Depending upon the various associated path losses of the DAU's, the central receiver may require a relatively wide dynamic range to accommodate all of the DAU's. It would be more desirable to have the power levels transmitted from the various DAU's adjusted prior to transmission of data therefrom such that the power level of the received signal from each of the DAU's at the central receiver is essentially the same, thus requiring a lower dynamic range for the central receiver. However, it is impractical to provide a manual adjustment since the DAU's are seldom accessible for this adjustment nor are the path losses consistent, since they may vary as a result of environmental conditions. In addition, the DAU's are susceptible to varying parameters due to temperature, aging of components, etc.
In view of the above disadvantages, there exists a need for a power leveling system which is remotely activated from a centralized point, thus yielding a more versatile system.